When a baby's lips are stimulated with the nipple, the baby turns its head in the direction of the nipple and opens its mouth wide with its tongue at the bottom of the mouth. The wide-open mouth of the baby can be used to insert as much of the breast as possible into the baby's mouth. A correct latch-on means that nipple, areola (which is the circular area such as the colored skin surrounding the nipple), and underlying breast tissue are drawn deeply into the baby's mouth. And the baby's upper and lower lips should be turned out, in other words, the baby's upper and lower lips are everted.
During normal suckling, the baby uses negative pressure to attach to the breast, draw the nipple and areola into its mouth to make a seal, and uses positive pressure to stimulate the breast; as a result, the negative pressure and positive pressure together draw the milk out of the breast. When a large piece of breast has been taken into the mouth, the nipple is essentially free from frictional movement against surfaces in the mouth.
When the baby has an inadequate amount of tissue in its mouth, for example, the baby only has the nipple in its mouth, the teat does not extend well into the mouth, but is drawn in and out of the mouth, causing friction against the tongue and gums, resulting in frictional damage of the nipple such as a sore or cracked nipple. At the same time, the incorrect latch-on also causes inadequate stimulation to the nipple and areola of the breastfeeding mother and the milk removal effectiveness and efficiency is influenced as a result. Besides, the baby's suck reflex is not stimulated properly and as a result he/she does not receive enough nutrients during feeding.
If the baby is latched onto the breast correctly, the mother is less likely to feel nipple pain, and the baby is likely to get more milk as the baby compresses the areola beneath which the milk sinuses, in other words, the reservoirs for milk, are located.